var arr1 = [1,2,3,4,5];
var arr2 = ['a','b','c'];
var len = arr1.length < arr2.length ? arr1.length:arr2.length;
var data = arr1.length > arr2.length ? arr1:arr2;
var arr3 = [];
for(var i=0; i<len; i++){
    arr3.push(arr1[i]);
    arr3.push(arr2[i]);
}
for(var j=len; j<data.length; j++){
    arr3.push(data[j]);
}
console.log(arr3);

